7
986 SCHOOL SCIENCE AND MATHEMATICS ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY BY HAROLD J. ABRAHAMS Simon Gratz High School, Philadelphia, Pennsylvania Inspection of high school laboratory manuals in chemistry reveals the fact that very little or no opportunity is generally provided for electrochemical experiments. It is quite probable, however, that first-hand experience with the chemical effects of the electric current can be of excellent value to high school students both because such studies tend to enhance their inter- est in chemistry and because they facilitate the comprehension of many of the basic concepts of this science. Furthermore, students who have conducted a few interesting electrochemical experiments, are apt to approach the study of electricity with greater anticipation. The experiments herein described have been developed with these thoughts in mind and have been used with about forty students over the last two years. The laboratory in which these experiments are carried out, is furnished with direct current. An inexpensive arrangement of wiring for six pairs of students, is shown in Figure 1. FIGURE 1 Recently the laboratory tables have been rewired, each with three polarized receptacles on either side, so as to make the six groups at each table entirely independent of each other. I. Electrolysis of Wafer. A. Preparation of Hydrogen and Oxygen.

ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

Embed Size (px)

Citation preview

Page 1: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

986 SCHOOL SCIENCE AND MATHEMATICS

ELECTROLYTIC EXPERIMENTS INHIGH SCHOOL CHEMISTRY

BY HAROLD J. ABRAHAMSSimon Gratz High School, Philadelphia, Pennsylvania

Inspection of high school laboratory manuals in chemistryreveals the fact that very little or no opportunity is generallyprovided for electrochemical experiments. It is quite probable,however, that first-hand experience with the chemical effectsof the electric current can be of excellent value to high schoolstudents both because such studies tend to enhance their inter-est in chemistry and because they facilitate the comprehensionof many of the basic concepts of this science. Furthermore,students who have conducted a few interesting electrochemicalexperiments, are apt to approach the study of electricity withgreater anticipation. The experiments herein described havebeen developed with these thoughts in mind and have beenused with about forty students over the last two years.The laboratory in which these experiments are carried out, is

furnished with direct current. An inexpensive arrangement ofwiring for six pairs of students, is shown in Figure 1.

FIGURE 1

Recently the laboratory tables have been rewired, each withthree polarized receptacles on either side, so as to make the sixgroups at each table entirely independent of each other.

I. Electrolysis of Wafer.

A. Preparation of Hydrogen and Oxygen.

Page 2: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

ELECTROLYTIC EXPERIMENTS 987

The apparatus is arranged as in figure 2. Care must be takento select test tubes of approximately the same size, as the varia-tion in diameters of six-inch tubes may often be great enough togive what appear to be incorrect quantities of hydrogen andoxygen.The electrodes consist of strips of hard-rolled platinum foil,

7X25 mm., mounted in glass connection tubing by platinumwires of 4 cms. length. The platinum wires are extended by be-ing soldered on to copper wires of length great enough to passthrough the open end of the glass tubing which measures about12 cms. in height.

iiu&m Rppmw rotiinE [ncmnsis of Vnm

FIGURE 2

A number of advantages are gained by the use of platinum.The wires leading to the electrodes may readily be encased inglass tubing and thus allow electrolysis to take place only fromthe electrode surface, so that none of the products can escapebeing collected. Furthermore, platinum electrodes do not takepart in secondary electrode processes and thus neither lose theirperiod of usefulness nor upset the quantitative relationship ofhydrogen and oxygen. Copper electrodes, for example, are opento the objection that they permit tremendous oxidation at theanode, if sodium hydroxide be used as electrolyte, with the re-sult of almost complete absorption of the oxygen. Lead elec-trodes in sulphuric acid would of course also be unsatisfactory.Such electrodes may however be used with sodium hydroxide,with fairly good results. In this case the electrode is cylindrical

Page 3: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

988 SCHOOL SCIENCE AND MATHEMATICS

in form, 1 cm. in diameter and 3 cms. high, giving a maximumexposed surface. However, with platinum at its relatively lowmarket price, the cost of equipping a laboratory with 18 pairsof such electrodes is not prohibitive. The use of copper extend-ing wires inside the glass tubing effects an appreciable saving.For electrolyte 0.5 to 1.0% sulphuric acid is very satisfactory,

giving rapid results and being very clear in appearance duringelectrolysis, so as to permit observation of the progress of thereaction. It is of course useless with pneumatic troughs made ofmetal or with imperfect enamel-ware.

Sulphuric acid may be replaced by sodium hydroxide of thesame strength, but the high surface tension of solutions of thelatter electrolyte cause such a frothing inside the test tubes thatthe progress of the electrolysis is greatly obscured. Furthermore,observations in this laboratory seem to indicate that theelectrolysis time is longer, using sodium hydroxide as electro-lyte.

Students in this laboratory, using the apparatus described,obtain a six-inch test-tube full of hydrogen in five minutes witha 100 watt lamp (in series) or in eight minutes with a 60 wattlamp, using sulphuric acid as electrolyte. Using sodium hydrox-ide, they obtain the same results in 7 minutes with a 100 wattlamp and in 9 minutes with a 60 watt lamp.A crude verification of the volumetric composition of water

may be made, by having students measure the volume of thegases in the tubes, at any time during the experiment. When thehydrogen tube is full, the tubes are removed and the gases inthem are tested for with flame and spark respectively.

B. Preparation of an explosive mixture of hydrogen and oxygen.The apparatus is set up again and allowed to proceed as be-

fore, until the hydrogen tube is two-thirds full. The polarity isnow reversed, by suitable exchange of wires at the Fahnestockclips of the lamp-bank. When both tubes are full, their contentsare, of course, identical, i.e., 2 volumes of hydrogen to 1 volumeof oxygen. A flame is applied to each tube.Among several values of this experiment is the vivid experi-

ence of the complete control which the chemist frequently hasover directing electrochemical processes.Another advantage is the sharpness of the report resulting

from the explosion�it is very easy for students to obtain thecorrect mixture and thus benefit by the satisfaction which a

Page 4: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

ELECTROLYTIC EXPERIMENTS 989

successfully performed experiment gives beginners in this sci-ence. As ordinarily performed, this experiment makes use of atest tube five-sevenths full of air and two-sevenths full of hydro-gen. The tube is thus less than half full of reacting gases. In theelectrolytic mixture the tube is entirely full of reactants.A third general outcome of such experiments is the better op-

portunity for student-teacher contact in the case of studentswith inclinations along electrochemical lines.

II. Electrolysis of Sodium Chloride solution.The apparatus is arranged as shown in figure 3. The cathode

is an iron "spike/7 mounted in a cork. The anode is a graphiterod, 10 cms. long and 1 cm. in diameter, inserted into a rubber

5TUDINT (IPPAEUTU5 f0d iHt titCTDOl/pIS

y SODIUM CruoDiDi ioiu^OMFIGURE 3

stopper. The stopper, in turn, is inserted into the flanged mouthof a bottomless test tube, made by cutting the upper 11 cms.from an 8 inch test tube, if not available from dealers. Theelectrolyte consists of a saturated (approximately 6 molar)solution of sodium chloride in water.

Using a 100 watt lamp, students in this laboratory obtaina tube of chlorine gas in about 15 minutes.During electrolysis a few drops of phenolphthalein indicator

are placed at the cathode to detect sodium hydroxide. Thestriking result obtained here seldom fails to influence the mostindifferent student. At the same time it gives confirmation to

Page 5: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

990 SCHOOL SCIENCE AND MATHEMATICS

the fact, previously learned in class, that lye is a by-product inthe manufacture of hydrogen and chlorine.At the conclusion of the electrolysis the tube is removed, a

small piece of moist paper, bearing newsprint, red ink and pen-cil markings is inserted and the tube then stoppered at bothends, to study the bleaching effects of chlorine.

It has also been found instructive to set up two demonstra-tion cells, using potassium bromide and potassium iodide, whichoperate while students are preparing their chlorine. These cellsdiffer from student apparatus only in that glass troughs, fittedwith asbestos diaphragms across the center, are substituted forpans. The concentrations are about 0.4 molar potassium brom-ide and 0.6 molar potassium iodide. Halogen forms in one com-partment and permits of the easier detection of potassiumhydroxide in the other compartment. Students thus becomefamiliar with the three halogen elements at one time and receive

5mm hfjiuiin fee I(IE fLtciooipisy CopPfR SULPHAJL 501UTK)N

FIGURE 4

an orientation to this family of elements. From their observa-tions on the contrast in colors of the free halogens and theirions, they also have a basis for recall, when later studyingthe theory of ionization, that ions and molecules of the sameelement differ from each other in appearance. The more alertstudents frequently suggest a halide-phenolphthalein mixturefor determining the polarity of a cell, after performing this ex-periment, showing that it has proved thought-provoking tothem.

III. Electrolysis of Copper Sulphate Solution.The apparatus is arranged as shown in figure 4.The electrodes are copper plates 3.5 cms. square. A perfora-

Page 6: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

ELECTROLYTIC EXPERIMENTS 991

tion, near the top, allows the plate to be suspended from thehook of the connecting wire, two of which are mounted in atwo-perforation stopper. The electrolyte is 15% Copper Sul-phate solution, containing a trace of free sulphuric acid. With a60 watt lamp in series, 0.200 gm. of copper may be depositedin less than 30 minutes. The experiment is made "quantitative"by dipping the plates in boiling distilled water, allowing themto drain off, then dipping again in two containers of alcohol andfinally in ether, draining again each time. They are then handledentirely with forceps, being weighed on a triple-beam balance.At the end of the experiment they receive the same treatmentand are weighed again.

Table I shows a few typical results obtained in this laboratory.

TABLE ICHANGE OF WEIGHT AT ELECTRODES

Experiment No. 1 2 3 4 5 6Gain at Cathodeingms. 0.450 0.550 0.478 0.500 0.540 0.500

Loss at Anodeingms. 0.470 0.520 0.520 0.470 0.520 0.500

These results tend to confirm the essential correctness of thestatement that the deposited metal is merely transferred fromthe anode in this experiment, the composition of the bath re-maining constant. Students thus have their knowledge of elec-tro-deposition of metals enriched.

TABLE IICONFIRMATION OF FARADAY^S LAW BY TYPICAL GROUPS

Quantity Approxi-Stu- ^. . . Gain at required by matedent- TVne, Aw^- Wattage Cathode Faraday’s ErrorGroup Mwl{fes a^ in gms. Law�W (%)

1 30 0.2 30 0.3 30 0.4 30 0,5 30 0.6 30 0,

,80 100 0,.90 100 0,.82 100 0,,81 100 0,.80 100 0,.87 100 0.

.450 0.475 -5

.550 0.535 +3

.478 0.487 -2

.500 0.481 +4

.460 0.475 -3

.500 0.517 -3

By placing an ammeter in series in the circuit, it is also pos-sible to use this experiment for teaching and verifying Fara-day’s Law. Table II shows a few results.

Page 7: ELECTROLYTIC EXPERIMENTS IN HIGH SCHOOL CHEMISTRY

992 SCHOOL SCIENCE AND MATHEMATICS

W (in table II) the weight of copper required by Faraday’sIXfXE

Law, was calculated by means of the relationship W ==����

96,500where I is the number of amperes flowing during time (inseconds) ^t^ and E is the equivalent weight of copper.At the conclusion of the quantitative phase of this experi-

ment, the polarity may be reversed and the cathode "stripped"though very small fragments may become detached from it. Inaddition to the theoretical values suggested above, this experi-ment may be used to illustrate three important processes:

1. The electrolytic refining of copper.2. The quantitative determination of copper.3. Copper plating.

SUMMARY

Electrochemical experiments for high school students arediscussed and very simple but satisfactory apparatus described.These experiments include the preparation of hydrogen, oxy-gen, chlorine, bromine, iodine and the deposition of copper. Inthe case of copper, the experimental work is made quantitative,and a crude verification of Faraday^s Law is made. A method ispresented for obtaining an oxygen-hydrogen mixture, whichgives vivid detonating results.

FROM GALILEO TO COSMIC RAYS�A BOOK REVIEW

By PHILIP A. CONSTANTINIDESCity College of Chicago, North Branch

From Galileo to Cosmic Rays, by Harvey Brace Lemon, Professor of Physics,The University of Chicago. Cloth. Pages xviii+450. 18.5 X23 cm. 1934.The University of Chicago Press, 5750 Ellis Ave., Chicago, 111. Tradeedition (with stereoscope) $5.00, Educational Edition §3.75, stereo-scope 75 cents extra.

Despite its rather unusual title, one does not get a real warning con-cerning the very unusual presentation of the material included in thisrecent addition to our scientific literature. The author in the openingparagraphs of his book, gives as his reason for this addition to so manyother excellent books on physics, a need originating from the nature andscope of the survey courses offered at the University of Chicago andhopes that it will meet the interests of the adult reading public.

After reading numerous sections of the book and examining the platesand sketches that not only help to illustrate, but also to complement andembellish the text, we feel, not only that the author is over modest butthat the reviewer on account of limitations of space will be handicappedin doing full justice to the book.